Integrated circuit (IC) functionality is commonly extended with sensor functionality for a wide variety of reasons. For instance, sensor functionality may be included in the form of moisture sensors or shock sensors to determine if a field return of the IC is caused by misuse of the apparatus in which the IC was integrated. Such sensors are typically not used in normal operation of the IC. However, the IC may equally comprise one or more sensors that are part of the operational functionality of the IC, for instance to measure the presence and/or concentrations of one or more analytes in a medium to which the IC is exposed, e.g. a sample or the atmosphere into which the IC is placed. Such ICs find their application in medical diagnostics as well as in other application domains.
The IC may for instance comprise radio-frequency identification tag functionality, which is used to identify the exposure of a product to which the IC has been tagged to one or more environmental factors, which for instance can be used to aid the appropriate storage of the tagged product, e.g. in case of perishable products, to extend its shelf life.
It is known that sensing elements can be advantageously integrated into the back-end of an IC manufacturing process, e.g. by integration of the sensing element into the metallization stack of the IC. Such a metallization stack typically provides the necessary interconnections of circuit elements mounted on the substrate of the IC, e.g. transistors, as well as the interconnections between the circuit elements and the outside world.
One of the challenges IC designers are faced with when integrating multiple sensors into the metallization stack is that environmental sensors have to be in communicative contact with the environment. This however increases the risk that moisture penetrates the metallization stack and interferes with the correct operation of the underlying circuit elements of the IC. To this end, a moisture barrier layer such as a Ta2O5 layer or a passivation layer may be deposited over the metallization stack to protect the underlying circuitry from exposure to moisture. However, when integrating multiple sensors with different functionality in the metallization layer, the partial removal of such a barrier layer cannot always be avoided.
An example of such a multi-sensor IC is shown in FIG. 1, in which an environmental sensor 120 having a transducer 122 and a shock sensor 140 are formed in a top metal layer of a metallization stack of the IC, the underlying layers of which are schematically depicted by a single layer 100 for the sake of clarity only. Such a top metallization layer may for instance comprise a stack of metal layers such as an AlCu layer 104 sandwiched between TiTiN layers 102 and 106. A bond pad 160 is also shown by way of example only.
An example of a shock sensor as disclosed in unpublished European patent application 09165533.2 is shown in FIG. 2. The shown cross-section comprises structures for a shock sensor on the left and a conventional bond pad 260 on the right. The entire die is capped by layers of oxide and nitride, the so-called passivation stack 250 or scratch protection, which protects the chip from external influences such as mechanical damage, e.g. scratches. The shock sensor is implemented as an inertial mass element 215 suspended by conductive connecting portions 240, which are connected to support structures 230 to electrically connect the inertial mass element 215 to active circuitry of the IC.
The inertial mass element 215 may be formed as a patterned metal portion of the metallization stack, which comprises of patterned conductive (metal) layers separated by dielectric layers indicated by reference numeral 210, which each dielectric layer typically comprising one or more conductive vias 220 interconnecting portions of different conductive layers with each other.
The conductive connection portions 240 will have an elastic modulus defined by the materials chosen to form these portions, which for instance may be formed as part of the metal layer to which they belong. Because of this elastic modulus, the conductive connection portions 240 act as springs when the inertial mass element 215 is suspended in a fluid such as air. The conductive connection portions 240 may be recessed with respect to the surrounding metal layer. This may be achieved by subjecting these portions to an additional etching step during the formation of the metallization stack. For instance, in case of multi-layered conductive connection portions 240, the metal stack of the portions can be reduced, e.g. etching away the top TiN and AlCu metal layer, leaving only a Ti/TiN stack. An advantage of this approach is that such a thin stack breaks more easily compared to a metal stack having its full thickness, thus allowing a reduction of the size of the inertial mass to achieve the same detection characteristics, i.e. the detection of the same acceleration force threshold being exceeded, which translates to a reduction in overall sensor size and cost.
Alternatively, the conductive connection portions 240 may consist of relatively narrow lines involving the same metal stack as the inertial mass element 215. In this embodiment, the entire shock sensor can be fully realized using conventional metallization stack processing followed by one additional process step.
In order to create a void or cavity 130 around the inertial mass element 104, the passivation layer 250 including the moisture-impenetrable layer 150 shown in FIG. 1 is removed from over the inertial mass element 215 by means of a conventional passivation etch, e.g. a bond pad opening etch. Openings are shown over the inertial mass element 215 and the bond pad 260. The etchant is selected such that it removes the dielectric materials but does not attack the conductive structures including inertial mass element 215 and the conductive connection portions 240, resulting in a ‘free floating’ inertial mass element 215 being obtained.
Now, returning to FIG. 1, a passivation stack comprising a moisture-impenetrable layer 150 such as a Ta2O5 layer, an oxide layer 112 and a nitride layer 114 covers the IC and protects it from damage, e.g. from moisture penetrating the IC and interfering with the correct operation of the active circuitry. However, the consequence of the formation of the cavity 130 in which the sensor 140 is placed means that the a moisture-impenetrable layer 150 has been disrupted, as such cavities are typically formed by providing a number of openings 116 in the nitride layer 114 above the sensor 140 and etching away the oxide 112 as well as the moisture-impenetrable layer 150 surrounding the sensor 140 to form the cavity 130.
Consequently, as the environmental sensor 120 has to be exposed to the environment, i.e. by opening the passivation stack above the transducer 122, a path exists for moisture to penetrate the IC through this opening, the relatively porous oxide layer 112 exposed in this opening and the cavity 130, as indicated by the arrow in FIG. 1. This is of course unwanted as such moisture penetration can detrimentally affect the operation of the IC as previously explained.